You need to initialize errorSoon, as indicated by the error message, you have only declared it.

Use with AIString[] errorSoon;                   // <--declared statement
String[] errorSoon = new String[100]; // <--initialized statement

You need to initialize the array so it can allocate the correct memory storage for the String elements before you can start setting the index.

If you only declare the array (as you did) there is no memory allocated for the String elements, but only a reference handle to errorSoon, and will throw an error when you try to initialize a variable at any index.

As a side note, you could also initialize the String array inside braces, { } as so,

Use with AIString[] errorSoon = {"Hello", "World"};

which is equivalent to

Use with AIString[] errorSoon = new String[2];
errorSoon[0] = "Hello";
errorSoon[1] = "World";

You have forgotten to give the variable a name? Both of these work.

String[][] array = {emptyArr, filledArr, moreFilledArr};

String[][] array = new String[][] {emptyArr, filledArr, moreFilledArr};

I recommend you to go through the basic Java syntax specification and tutorials. Start with The Java Tutorials by Oracle Corp, free of cost.

As others have said,

new String[0]

will indeed create an empty array. However, there's one nice thing about arrays - their size can't change, so you can always use the same empty array reference. So in your code, you can use:

private static final String[] EMPTY_ARRAY = new String[0];

and then just return EMPTY_ARRAY each time you need it - there's no need to create a new object each time.

You could do something like this

String[] myStrings = { "One", "Two", "Three" };

or in expression

functionCall(new String[] { "One", "Two", "Three" });

or

String myStrings[];
myStrings = new String[] { "One", "Two", "Three" };